1. Field of the Invention
The present disclosure relates to a location awareness technology, and more particularly, to an apparatus and method for collaborative location awareness based on weighted maximum likelihood estimation (MLE), which is capable of improving accuracy of location awareness between nodes in estimating a location of a blind node.
2. Description of the Related Art
Neal Patwari proposed the Cramer-Rao bound (CRB) and the maximum-likelihood estimator (MLE) for location awareness using time-of-arrival (TOA) information and received signal strengths (RSS) information between a sensor and its peripheral sensors in “Relative location estimation in wireless sensor networks, IEEE Trans. Signal processing, vol. 51, no. 8, pp. 2137-2148, August 2003 by N. Patwari, A. O. Here III, M. Perkins, N. S. Correal, and R. J. O'Dea”.
In an algorithm proposed by N. Patwari, the MLE of the following Equation (1) is used as an estimate of a location of a blind node.
                                          θ            ^                    R                =                  arg          ⁢                                          ⁢          min          ⁢                                    ∑                              i                =                1                                            m                +                n                                      ⁢                                          ∑                                                      j                    ∈                                          H                      ⁡                                              (                        i                        )                                                                                                  j                    <                    i                                                              ⁢                                                (                                      ln                    ⁢                                                                                                                        ⅆ                            ~                                                                                i                            ,                            j                                                    2                                                ⁢                                                  /                                                      C                            2                                                                                                                      ⅆ                                                  i                          ,                          j                                                2                                                                              )                                2                                                                        (        1        )            where,    {circumflex over (θ)}R: coordinate estimate of a blind node using RSS information,    di,j: Euclidean distance,    {tilde over (d)}2i,j: distance calculated based on the path-loss model by using the measured RSS information,    m: number of reference nodes,    n: number of blind nodes,    H(i): nodes linked with node i, and    C: defined as the following Equation (2)
                    C        =                  exp          ⁡                      [                                          1                2                            ⁢                                                (                                                                                    ln                        ⁢                                                                                                  ⁢                        10                                            10                                        ⁢                                                                  σ                        dB                                            n                                                        )                                2                                      ]                                              (        2        )            
{circumflex over (θ)}R, which is the estimate that minimizes a maximum likelihood (ML) function expressed by the above Equation 1, is used an estimate of a location of blind nodes.
In the location awareness technology proposed in the algorithm by N. Patwari, the location estimation is performed basically in equal consideration of reference-node information and blind-node information. However, an error is not calculated in this algorithm despite the fact that the coordinates of the estimated location of the blind node include an error. That is, even though the related art location awareness algorithm such as the algorithm by N. Patwari includes an error of the location estimation in using the blind-node information, the error is not considered therein, which results in sensitive reaction to a fading situation of a location estimation environment and in deterioration of entire location estimation performance. Particularly, severely errored location-estimation may be caused in a specific fading environment.